1. Field of the Invention
The present invention relates to the field of microelectronics, in particular it relates to an apparatus for detecting flatness of a top surface of a wafer and the detecting method thereof.
2. Description of the Prior Art
In the production process of integrated circuits, the surface of the silicon wafer is usually not perfectly flat but is undulating. If the undulating surface is ignored, it will cause partial regions of the exposed areas to be out of focus in the exposure process of traditional photolithography so that the image of the pattern will be affected.
The detection for wafer flatness by a scanner is an important measurement step prior to the exposure of the wafer. Because of this measurement step, the scanner will expose the film such as photo-resist covered on the wafer under the optimal exposure conditions corresponding to the surface of the wafer. The surface with the smallest overall differences in surface irregularities becomes the best focused plane at the various points from the exposure area and is called the best focused surface.
Currently, there are two methods in the present industry for detecting the flatness of a wafer by a scanner,
1. The optical method: emitting light onto the surface of the wafer, then the light is reflected by the surface of the wafer and the reflected light is received by a sensor. The intensity of the reflected light is determined by the sensor, and the height of the point calculated. A row of equally spaced groups of light transmitters and light receivers are arranged parallel in the lateral direction, then the wafer is scanned in the longitudinal direction. As a result, the height of every point on the wafer can be obtained, which is used for determining the flatness of the entire wafer.
In the process of this method, since the emitted light will penetrate the photoresist so that what the method measures is the flatness of the substrate. During the process of coating the photoresist, the photoresist flows before it is heated and dried, thus there are some differences between the flatness of the substrate and the flatness of the photoresist. However what we actually want to measure is the flatness of the photoresist surface. Therefore, the flatness measured by this method is somehow different from the actual desired flatness.
2. The (Air Gauge) method: This method combines the optical method and air method. Firstly, the optical method is applied to measure the height of each point on the wafer surface in the exposed area. Then air is sprayed on a point within the exposed area on the surface of the wafer. A sensor receives the air rebounding from the surface of the wafer. According to the different flow rates of the air received by the sensor, the height of the point is calculated. Then the different points within the exposed area are detected to determine the height of every point in the exposed area. The difference of every point which is detected between the air measurement and the optical measurement will be calculated. As the substrate height is different at various points within the exposed area, the difference values of these points will not the same. A model is built based on these difference values, which is recorded in the scanner. In the future actual production, the optical detection method is firstly used to measure the flatness of the wafer, then this model is called and the difference value is added point-to-point, the result will be equivalent to the flatness measured by air.
As using the air method for measurement is relatively slow and the flows will affects each other if a row of such air jets and sensors are arranged, the air method will not work alone and the optical method has to be combined to detect the flatness. If the combined method is used to detect the different substrates, different models need to be established according to the difference values between the measured results in air flow approach and the measured results using the optical method for each product each layer. Therefore, the model establishment used in production will result in a relatively large workload. Once the substrate changes, the flatness measured by this method will not be accurate and the data will need to be collected again to build a new model. In practical production, because of the shift in the etching or polishing process, measurements inaccuracies will result.
Chinese patent (CN 101276151A) discloses a method and device for measuring the smoothness of a wafer surface. The morphology values of the different positions of the wafer surface are measured by the redundancy of the vertical location device of the work piece stage in an exposure device. The smoothness at any point on the wafer surface is acquired through the polynomial fitting calculation. The technical solution described in the literature has not solved the problem of the inaccuracy in measurement caused by the bias in the etching or polishing process.
Chinese patent (CN 101957186A) describes a method for detecting the flatness of the wafer surface. A light that is parallel to the surface of the wafer is used to irradiate outside, and the wafer surface flatness is detected according to the direction of the beam received by the light receiver.